With the development of technology, electrical power supply devices have been getting wider and wider applications. More and more products incorporate the power supply device, especially for personal computers, industrial computers, exchange devices, cash registers, printers, which all need alternate current to direct current power conversion and use a switching construction to turn on/off the power.
The currently available switching power supply device often adopt pulse width modulation (PWM) type control for output of direct current for the reason of reducing the size of transformer. An example is shown in FIG. 1, which illustrates a circuit diagram of a conventional forward circuit. A transformer T1 splits a circuit zone into a primary circuit on a primary side and a secondary circuit on a secondary side. PWM control means on the primary circuit of the primary side is connected to the secondary circuit of the secondary side so that a voltage feedback is returned from the secondary circuit to adjust the duty cycles of two ON/OFF switches S1, S2 on the primary side to realize stable output of voltage.
By the switching operation of the two switches S1, S2, electrical power stored in a capacitor can be transmitted through a coupling capacitor to the primary side of the transformer T1 to serve as an alternate current power source. The alternate current power source provides electrical energy to the transformer T1 and the transformer T1 transforms the energy to the secondary side. The secondary side of the transformer is coupled through a rectification unit to an energy accumulation inductor Lo and then to a load. When both switches S1, S2 are open or turned off, the energy accumulation inductor L0 releases energy to supply the required electrical power, and a capacitor C1 is used to provide filtering of direct current. The secondary side is connected to diodes D1, D2 to convert alternate current waveform into direct current waveform. The diodes D1, D2, when conducted on, cause a voltage drop of 0.4V-1.5V. Thus, when current output is significant, a great loss of power may occur, leading to poor efficiency.
To conclude, the above described architecture of the conventional circuit is disadvantageous in potential risk of great loss of power and consequently poor efficiency.
Thus, the present invention is aimed to provide a circuit architecture wherein a synchronous rectification control circuit is coupled to the secondary-side rectification circuit to reduce the conduction-on power loss.